1. Field of the Invention
The present invention relates in general to the hydroprocessing of products from a Fischer-Tropsch synthesis reaction. More specifically, embodiments of the present invention are directed toward an acid extraction process for effectively removing contaminants, fouling agents, and/or plugging precursors from the Fischer-Tropsch hydrocarbon stream prior to sending that product stream to a hydroprocessing reactor.
2. State of the Art
The majority of the fuel used today is derived from crude oil, and crude oil is in limited supply. However, there is an alternative feedstock from which hydrocarbon fuels, lubricating oils, chemicals, and chemical feedstocks may be produced; this feedstock is natural gas. One method of utilizing natural gas to produce fuels and the like involves first converting the natural gas into an “intermediate” known as syngas (also known as synthesis gas), a mixture of carbon monoxide (CO) and hydrogen (H2), and then converting that syngas into the desired liquid fuels using a process known as a Fischer-Tropsch (FT) synthesis. A Fischer-Tropsch synthesis is an example of a so-called gas-to-liquids (GTL) process since natural gas is converted into a liquid fuel. Typically, Fischer-Tropsch syntheses are carried out in slurry bed or fluid bed reactors, and the hydrocarbon products have a broad spectrum of molecular weights ranging from methane (C1) to wax (C20+).
The Fischer-Tropsch products in general, and the wax in particular, may then be converted to products including chemical intermediates and chemical feedstocks, naphtha, jet fuel, diesel fuel, and lubricant oil basestocks. For example, the hydroprocessing of Fischer-Tropsch products may be carried out in a trickle flow, fixed catalyst bed reactor wherein hydrogen (H2), or a hydrogen enriched gas, and the Fischer-Tropsch derived hydrocarbon stream comprise the feed to the hydroprocessing reactor. The hydroprocessing step is then accomplished by passing the Fischer-Tropsch derived hydrocarbon stream through one or more catalyst beds within the hydroprocessing reactor with a stream of the hydrogen enriched gas.
In some cases, the feeds to be hydroprocessed contain contaminants that originate from upstream processing. These contaminants may take either a soluble or particulate form, and include catalyst fines, catalyst support material and the like, and rust and scale from upstream processing equipment. Fischer-Tropsch wax and heavy products, especially from slurry and fluid bed processes, may contain particulate contaminants (such as catalyst fines) that are not adequately removed by filters provided for that purpose. The removal of those particulates prior to hydroprocessing may be complicated by the potentially high viscosities and temperatures of the wax stream leaving the Fischer-Tropsch reactor.
The typical catalyst used in a hydroprocessing reactor demonstrates a finite cycle time; that is to say, a limited time (or amount) of usefulness before it has to be replaced with a new catalyst charge. The duration of this cycle time usually ranges from about six months to four years or more. It will be apparent to one skilled in the art that the longer the cycle time of a hydroprocessing catalyst, the better the operating efficiency of the plant.
Soluble and/or particulate contaminants can create serious problems if they are introduced into the hydroprocessing reactor with the feed. The soluble contaminants pose a problem when, under certain conditions of hydroprocessing, they precipitate out of solution to become particulates. The contamination can cause partial or even complete plugging of the flow-paths through the catalyst beds as the contamination accumulates on the surfaces and interstices of the catalyst. In effect, the catalyst pellets filter out particulate contamination from the feed. In addition to trapping debris that is entrained in the feed, the catalyst beds may also trap reaction by-products from the hydroprocessing reaction itself, an example of such a reaction by-product being coke. Plugging can lead to an impairment of the flow of material through the catalyst bed(s), and a subsequent buildup in the hydraulic pressure-drop across the reactor (meaning the pressure differential between the ends of the reactor where the entry and exit ports are located, respectively). Such an increase in pressure-drop may threaten the mechanical integrity of the hydroprocessing reactor internals.
There are at least two potentially undesirable consequences of catalyst bed plugging. One is a decrease in reactor throughput. A more serious consequence is that a complete shut down of the reactor may be required to replace all or part of the catalyst charge. Either of these consequences can have a negative effect on operating plant economics.
Prior art attempts to manage the problem of catalyst bed plugging in hydroprocessing reactors have been directed toward eliminating at least a portion of the particulate contamination in the feed by filtering the feed prior to its introduction to the hydroprocessing reactor. Such conventional filtration methods are usually capable of removing particulates larger than about 1 micron in diameter. Other prior art methods have been directed toward either controlling the rate of coking on the hydroprocessing catalyst, selecting a feed that is not likely to produce coke, or judiciously choosing the hydroprocessing conditions (conditions such as hydrogen partial pressure, reactor temperature, and catalyst type) that affect coke formation.
The present inventors have found, however, that the above-mentioned open art methods are not effective at removing very small sized particle (or soluble) contaminants, fouling agents, and/or plugging-precursors (hereinafter referred to as “contamination”) from the feedstream to a hydroprocessing reactor when that feedstream comprises a Fischer-Tropsch derived hydrocarbon stream. This is particularly true when the Fischer-Tropsch derived hydrocarbon stream is a wax produced by a slurry bed or fluid bed process. Typical open art methods have therefore not been found to be effective at avoiding the pressure-drop buildup in a hydroprocessing, hydroisomerization, or hydrotreating reactor when that buildup is caused either by particulate contamination, or by soluble contamination that precipitates out of solution.
The apparent failure of typical open art methods has been attributed to either the presence in the hydroprocessing reactor feed of finely divided, solid particulates with diameters of less than about 1 micron, and/or to a soluble contaminant, possibly having a metallic component, with the ability to precipitate out of solution adjacent to or within the hydroprocessing reactor catalyst beds. What is needed is a method of removing particulates, contaminants, soluble contamination, fouling agents, and plugging precursors from the feedstream to a hydroprocessing reactor such that pressure drop buildup within the hydroprocessing reactor is substantially avoided.